Recovery of durene by forming a cocrystallized solid with symmetrical tetrachloroethane



United States Patent RECOVERY OF DURENE BY FORMING A CO- CRYSTALLIZED SOLID WITH SYMMETRI- CAL TETRACHLOROETHANE Delos Edward Bown, Baytown, Tex., assignor, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, N. J., a corporation of Delaware Application June 17, 1954, Serial No. 437,532

5 Claims. (Cl. 260-674) The present invention is directed to a method for separating durene from durene-containing fractions. More particularly, the invention is concerned with the improved method of recovering durene from an admixture with other aromatic hydrocarbons. In its more specific aspects, the invention is directed to separation of durene by co-crystallization with symmetrical tetrachloroethane.

The present invention may be briefly described as a method for recovering durene from a durene-containing hydrocarbon fraction which comprises admixing said fraction with symmetrical tetrachloroethane in an amount of at least 20% by volume of said fraction following which the admixture is chilled to form a co-crystallized solid of the durene and the tetrachloroethane in a mother liquor. The crystalline solid is then separated from the mother liquor and the solid is melted and may be distilled to recover durene from the tetrachloroethane.

In accordance with the present invention, the symmetrical tetrachloroethane is admixed with the durenecontaining hydrocarbon fraction in an amount greater than by volume of the durene-containing fraction. Amounts of the symmetrical tetrachloroethane may be used in the range from about 20% to about 40% by volume of the durene containing fraction. Satisfactory results are obtained with about 30% by volume of the tetrachloroethane.

Temperatures to be employed in the practice of the present invention for forming the co-crystallized solid of durene and tetrachloroethane may suitably range from about -20 to about 80 F. with temperatures intermediate the lowest and highest temperatures being preferred. A temperature of about 40 to about 60 F gives satisfactory results.

In practicing the present invention the chilled mixture may be held for a suitable time, which may be in the range from about minutes to about 4 hours, for formation of the co-crystallized solid. It may be desirable to chill the admixture originally to a temperature somewhat below the final crystallization temperature and permit it gradually to warm up to the final crystallization temperature.

The durene-containing hydrocarbon fraction may be obtained from any source and specifically may be obtained from catalytic cracking operations or from hydroforming operations which are well known to the art. An aromatic solvent fraction boiling in the range from about 355 to 450 F. may be the durene-containing fraction. Suitably a fraction containing aromatic hydrocarbons having from 9 to 11 carbon atoms in the molecule may be used.

The durene-containing fraction may boil in the range from about 300 to about 450 F. and may be distilled to separate a fraction boiling in the range from 380 to 390 F., or somewhat wider boiling fraction from which durene will be recovered by crystallization according to the present invention. Durene m y be satisfac' ice torily recovered from solvent naphthas of the type mentioned supra.

The invention will be further illustrated by reference to the drawing in which the single figure is a flow diagram of a preferred mode.

Referring now to the drawing, numeral 11 designates a line leading into the system by way of which a durenecontaining fraction is introduced into a distillation zone generally indicated by the numeral 12. The distillation zone may comprise a plurality of distillation towers, each containing suitable internal vapor-liquid contacting means, such as bell cap trays and the like, for precise separation of the hydrocarbons contained in the feed introduced by line 11. In this particular instance, zone 12 is illustrated by a single distillation tower which will be understood to comprise all auxiliary equipment usually associated with the modern distillation tower. Distillation zone 12 is provided with line 13 for separation of lighter fractions, line 14 for discharge of heavier fractions and with line 15 for separation of aromatic fractions containing durene, such as one containing from 9 to 11 carbon atoms in the molecule. A heater means, such as a steam coil 16, is provided in zone 12 to supply suflicient heat for separation of the hydrocarbon feed and for recovery of the fraction in line 15.

The durene-containing fraction in line 15 has admixed with it an amount of symmetrical tetrachloroethane as indicated before which is introduced into line 15 by line 17 from a source which will be described further.

The admixture in line 15 of the tetrachloroethane and the durene fraction then flows by way of line 15 into a suitable chilling means which may be a scraped surface chiller 18 of a type well known to the art. In chilling zone 18 the temperature of the mixture is reduced to a temperature in the range of about 20 to about F. to form a slurry of a mixture of solid durene and cocrystallized solids of durene and tetrachloroethane in a mother liquor. The slurry is then discharged from zone 18 after being chilled, as has been described, by way of line 19 into a holding tank 20 where the slurry may be held for a time within the range indicated while warming slightly, if desired, by means not shown.

After the holding time, as indicated, the slurry is discharged from the holding tank by way of line 21 controlled by valve 22 into a separation means illustrated by a centrifuge 23 which may be operated at a sufficient speed to separate a cake of crystallized solids and a filtrate or mother liquor. The mother liquor or filtrate is discharged from the centrifuge 23 by way of line 24 and may be discharged by opening valve 25 into a distillation zone 26 which is similar to distillation zone 12.

In the distillation zone 26 any tetrachloroethane present in the mother liquor is separated by heating and distilling off, the tetrachloroethane, having a lower boiling point than the durene, being withdrawn by line 27 and recycled to line 17 on opening valve 28.

The filtrate may discharge by way of line 24 into branch line 29 controlled by valve 30 for discharge from the system, if desirable.

When the filtrate is discharged into distillation zone 26, the aromatic hydrocarbons may be discharged from zone 26 by line 35 or a separation may be made between the hydrocarbons, taking oflf a lighter fraction through line 36 controlled by valve 37 with heavier materials being discharged by line 35.

The cake of crystallized solids of durene and tetrachloroethane is discharged from centrifuge 23 by line 38 which contains a heating means 39 which serves to melt the complex. Since the tetrachloroethane melts at about -43.8 C. and since durene melts at 78 to 80 C. it will be desirable to provide suflicient heat by heater 39 to melt the solid. The melt then discharges byline 38 into distillation zone 40 which is similar to zones 26 and 12. In distillation zone 40 conditions are adjusted byz heating means .41 which suitably may the a steam coil to distill the tetrachloroethane .fromthe durene. Tetrachloroethane boils at about 295 F. and therefore, may be recovered overhead by line 42 and discharged thereby into line 17 for re-introduction into line 15,;the purified durene being recovered from zone 40 by line 43.

Since some of the tetrachloroethane may be lost in the mother liquor, it may be desirable from time to time to introduce additional symmetrical tetracholoroethane as makeup into the system by opening valve 44 in line 45 which connects into a source of the tetrachloroethane not shown.

'In order to illustrate the invention further, a solvent naphtha boiling in the range between 355 and 450 F. which contained 9.6% by weight of durene was admixed with varying amounts of the symmetrical tetrachloroethane. In several runs which are represented in Table I, the mixture of the solvent naphtha and tetrachloroethane was cooled to 60 F. and then warmed to 40 F. and thereafter held at 40 F. for one hour before separating the crystals from the mother liquor. The mother liquor was analyzed for content of durene.

v. percent S-CHClz CHOlz v. percent s-CHClz CHCh i. v. percent S-CHClz CHClz From these results, it will be seen that 10 volume percent of the tetrachloroethane did not give any improvement and in fact was deleterious while 20 and 30 parts by volume of the tetrachloroethane decreased the amount of durene remaining in the mother liquor to substantially less than when no tetrachloroethane was added.

A specific feature of the present invention is that the symmetrical tetrachloroethane and the durene appear to form a complex crystalline solid which on melting will allow the durene to be separated therefrom by distillation. This is quite advantageous in that the tetrachloroethane may be recovered for reuse in the process and purified durene obtained in greater yield than would otherwise be possible.

It is not understood why the symmetrical tetrachloroethane is so eflective but it is believed that a complex crystalline solid is formed with the durene. It is known that some brominated methanes, specifically dibromodichloromethane and tetrabromomethane appear to form complexes with durene, but they are unsatisfactory for use in recovery of durene according to the present process since the brominated methanes are decomposed by heat, resulting in loss of these materials. Other halogenated compounds are also unsuitable since they do not form complexes with durene; for example, monobromodichloromethane does not form a complex or cocrystallized solid with durene.

'It will be seen from the foregoing description and the examples that a new and improved method of separating durene has been provided which is of considerable utility since the durene may be separated in substantial purity and the symmetrical tetrachloroethane recovered by simple distillation.

The nature and objects .of the present invention having been completely described and illustrated, what I wish to claim as new and useful and to secure by Letters Patent is:

1. A method for recovering durene from a durene-containing hydrocarbon fraction which comprises admixing said fraction with symmetricaltetrachloroethane in an amount ,at least 20%-by? volume, chilling the admixture to form a co-crystallizedrsolid-of said durene and said tetrachloroethane in a mother liquor, separating said cocrystallized solid from sa id"mother liquor and recovering durene from said co cry'sjt alliz ed solid.

2. A method for recovering durene from a durenecontaining hydrocarbon fraction which comprises admixing said fraction with symmetrical tetrachloroethane in an amount at least 20% by volume, chilling the admixture to form a co-crystallized solid of said durene and said tetrachloroethane in a' motherliquor, separating said co-crystallized a solid from said'- mother liquor and melting said co-crystallizedisoli'd -and distilling-to recover said durene.

3. .A method .for recovering durene from a durenecontaining shydrocarbon'ifraction which comprises admixing .said fraction with symmetricaltetrachloroethane in an amount at least 20% by volume, chilling the admiture to ;a temperature'zin the rangeibetween about 20 toabout .-..80 F. .toform a co orystallized solid of said durene and .said tetrachloroethane in a-mother liquor, separating; said corcrystallizedsolid: from said mother liquor, -and meltingrsaid covcrystallized solid and distilling to recover saiddurene.

4. A method 'forrecoveringdurene from a durenecontaining hydrocarbon fraction which comprises admixing said fractionwwith 'symmetricalutetrachloroethane in an amount inthe range between about 20% and about 40% byqvolume,,chillingthe admixture to a temperature in therange betweenqabout .'.40 .to about .80 F. to

form a co crystallized solid of said.durene.-and.sa id tetrachloroethane in a'motherwliquor, separating said complex crystalline solid from-said mother liquor, and melting said co-crystallized solid and "distilling .to recover said durene.

5. A method for recovering durene from an aromatic hydrocarbon fraction containing aromaticscontaining 9 to 11 carbon atoms and durene which comprises admixing said fraction vwith symmetrical tetrachloroethane in an amount in the range between about 20% and about 40% by volume of said fraction, chilling the admixture to a temperature of about 60 F. to form a co-crystallized solid of said durene and said tetrachloroethane in a mother liquor, Warming the chilled admixture to about 40 F. and holding at that temperature for about one hour, separating said co-crystallized solid from said mother liquor and then melting said co-crystallized solid and distilling to recover said durene.

References Cited in the file of this patent UNITED STATES PATENTS 2,665,316 Bennett Jan. 5, 1954 FOREIGN PATENTS 677,368 Great Britain Aug. 13, 1952 OTHER REFERENCES International Critical Tables, vol. IV, page (1928), McGraw-Hill Book Co., New York, N. Y.

Egan et al.: Industrial and EngineeringChemistry, vol. 47, No. 2, February 1955, pages 250-253. 

1. A METHOD FOR RECOVERING DURENE FROM A DURENE-CONTAINING HYDROCARBON FRACTION WHICH COMPRISES ADMIXING SAID FRACTION WITH SYMMETRICAL TETRACHLOROETHANE IN AN AMOUNT AT LEAST 20% BY VOLUME, CHILLING THE ADMIXTURE TO FORM A CO-CRYSTALLIZED SOLID OF SAID DURENE AND SAID 